Time-of-flight (TOF) systems are known in the art, and include both non-phased based systems such as described in U.S. Pat. No. 6,323,942 “CMOS-Compatible Three-Dimensional Image Sensor IC” (2001), and phase-based systems such as described in U.S. Pat. No. 6,580,496 “Systems for CMOS-Compatible Three-Dimensional Image Sensing Using Quantum Efficiency Modulation” (2003), which patent is incorporated herein by reference as further background material.
FIG. 1A exemplifies a phase-based TOF system 100, for example a system such as described in U.S. Pat. No. 6,580,496. TOF system 100 can be implemented on a single IC 110, without moving parts and with relatively few off-chip components. System 100 includes a two-dimensional array 130 of detectors (or sensors) 140, each of which has dedicated circuitry 150 for processing detection charge output by the associated detector. Collectively a detector 140 and its circuitry 150 comprise a pixel 155. In a typical application, array 130 might include 100×100 pixels 155. IC 110 also includes a microprocessor or microcontroller unit 160, memory 170 (which preferably includes random access memory or RAM and read-only memory or ROM), a high speed distributable clock 180, and various computing and input/output (I/O) circuitry 190.
Under control of microprocessor 160, an oscillator 115 causes a source of optical energy 120 to be periodically energized and emit optical energy Sout via lens 125 toward an object target 20. Typically the optical energy is light, for example emitted by a laser diode or LED device 120. Sout preferably is a periodic signal with modulation frequency components of perhaps 200 MHz. For convenience, Sout may be represented as A·cos (ωt). Sout typically has low average and peak power in the tens of mW range, which enables emitter 120 to be an inexpensive light source with a relatively narrow bandwidth, e.g., a few hundred KHz. Some of the emitted optical energy Sout will be reflected off the surface of target object 20 as returning energy Sin, which may be represented as Sin=A·cos (ωt+φ), where φ is relative phase shift. Returning energy Sin passes through an aperture field stop and lens, collectively 135, and falls upon two-dimensional array 130 of pixel detectors 140 where an image is formed. Note that Sin may include ambient energy components in addition to the actively emitted Sout components.
Each pixel 155 measures intensity (or amplitude) of received Sin, and the relative phase shift (φ) between received Sin and emitted Sout, representing a measure of the roundtrip travel distance Z between system 100 and target object 20. For each pulse of optical energy transmitted by emitter 120, a three-dimensional image of a portion of target object 20 is acquired, where phase shift (φ) is analyzed to determine distance Z.
Emitted optical energy Sout traversing to more distant surface regions of target object 20 before being reflected back toward system 100 will define a longer time-of-flight than radiation falling upon and being reflected from a nearer surface portion of the target object (or a closer target object). In addition, different values for distances Z will manifest as different magnitudes of relative phase shift (φ). Thus, relative phase shift phase (φ) can provide a measure of the distance Z between system 100 and the target object 20. Detection of Sin signals over multiple locations in pixel array 130 results in measurement signals that are referred to as depth images. The acquired data includes luminosity data (e.g., signal amplitude A), and true TOF relative phase shift (φ), to determine distance Z values to surface regions of target objects 20.
In system 100′ there will be a phase shift φ due to the time-of-flight (TOF) required for energy transmitted by emitter 120 (S1=cos (ωt)) to traverse distance z to target object 20, and the return energy detected by a photo detector 140′ in array 130′, S2=A·cos (ωt+φ), where A represents brightness of the detected reflected signal and may be measured using the same return signal that is received by the pixel detector. FIGS. 1B and 1C depict the relationship between phase shift φ and time-of-flight, assuming for ease of description a sinusoidal waveform with period T=2π/ω.
The phase shift φ due to time-of-flight is:φ=2·ω·z/C=2·(2πf)·z/C 
where C is the speed of light 300,000 Km/sec. Thus, distance z from energy emitter (and from detector array) to the target object is given by:z=φ·C/2ω=φ·C/{2·(2πf)}
Various techniques for acquiring and processing three dimensional imaging data acquired TOF systems are known in the art. For example, U.S. Pat. No. 6,522,395 (2003) to Bamji et al. discloses Noise Reduction Techniques Suitable for Three-Dimensional Information Acquirable with CMOS-Compatible Image Sensor ICs.
The effective illumination provided by Sout as seen by target object 120 varies inversely with the square of Z. Thus, increasing magnitude of output power from emitter 120 can enhance system 100 performance, providing more accurate measurements over increasing magnitudes of Z. However in some systems emitter 120 may be bonded to IC 110, such that replacing the emitter with a more powerful (higher wattage) device may be difficult.
Thus, there is a need for a method by which one or more additional optical sources could be provided to augment intensity of Sout illumination as seen by the target object. Such additional sources could include relatively high powered emitter(s) located perhaps adjacent to system 100, and/or emitter(s) of less power located closer to the target object than the TOF primary source of optical power. However proper operation of the resultant system dictates that optical energy from each additional source be synchronized with optical energy Sout.
The present invention provides a method and system to provide at least one additional optical source that is synchronized with the optical energy generated by emitter 120 as source energy Sout. Such additional optical source(s) may be wireless synchronized to the TOF system primary optical source, and/or may be removably attached to the TOF system housing and thus be wired rather than wireless.